An experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease is critical for the study of human AIDS. SIV induces an immunodeficiency syndrome in infected macaques that is remarkably similar in pathogenesis to human AIDS. The purpose of this project is to investigate host and viral factors involved in variable disease progression in SIV-infected macaques and the lack of disease in African primates infected with their own strains of SIV. ROLE OF TRIM5 ALPHA IN SIV PATHOGENESIS. To investigate the role of host factors in SIV-infection of macaques, we used a well-defined molecularly cloned virus (SIVsmE543-3), two macaque passages from the original sooty mangabey host. Our studies identified that allelic polymorphisms in the SPRY domain of TRIM5 alpha gene are responsible for much of the inter-individual variation in viremia. In contrast, SIVmac appears to be adapted for resistance to rhesus macaque TRIM5. We evaluated the effect of restrictive TRIM5 alleles (TRIM-TFP and TRIM-CypA) on viremia in SIVsmE543-3-infected rhesus macaques. Restrictive genotypes were associated with significantly lower viremia than in macaques with the permissive genotype, and with emergence of escape mutations in the SIV capsid protein. Two amino acid substitutions (P37S and R98S) in the capsid region were associated with escape from TRIM5TFP restriction. Introduction of these mutations into the original SIVsmE543-3 clone resulted in escape from TRIM5 restriction in vitro and improved virus fitness in macaques with homozygous restrictive TRIM alleles in vivo. Introduction of these two capsid mutations to the related SIVsmE660-FL14 clone also conferred escape from TRIM5 restriction demonstrating common escape pathways from TRIM restriction in related SIV strains. NEUROAIDS. SIV and HIV are both associated with the development of encephalitis. For HIV, the onset of AIDS dementia is generally a late stage finding. In contrast, most models of SIV encephalitis (SIVE) use animals that progress rapidly to disease. We have performed sequential intravenous passage of virus isolated from the brain of rhesus macaques with SIV encephalitis and derived a viral swarm, SIVsmH804E that induces SIV meningitis and/or encephalitis at high frequencies. We assessed viral populations in the meninges and the brain parenchyma by laser capture microdissection and observed compartmentalization of viral populations between the meninges and the parenchyma. A distinguishing feature of this neurovirulent isolate is the ability to replicate efficiently in monocyte derived macrophages (MDM). We identified substitutions in the cytoplasmic tail of the envelope gp41 protein that are associated with enhanced replication in MDM and have shown that these substitutions also enhance the ability of envelope of this virus to counteract the host restriction factor, BST-2. We have derived a molecular clone from the neurovirulent isolate, SIVsm804E-757CL that induces neuroAIDS in a high proportion of rhesus macaques with permissive TRIM5 alleles. In addition we have introduced the Gag capsid mutations identified as conferring resistance to TRIM5 inhibition into this clone and are assessing whether this new virus is capable of inducing neuroAIDS in macaques with restrictive TRIM5 genotypes. This would give us more flexibility in designing studies of neuroAIDS. Finally we are evaluating mononuclear populations isolated from the brains of macaques with and without encephalitis and using cell sorting and quantitative PCR for SIV to identify cell types infected in the brain. Preliminary findings revealed SIV DNA within CD4+ T lymphocytes of the brain in all SIV-infected animals but only within brain macrophages in animals with SIV encephalitis. ASYMPTOMATIC INFECTION OF NATURAL HOST SPECIES. A second goal of this project is to study the mechanisms underlying the lack of pathogenicity of SIV for their natural host species, with emphasis on SIVagm from vervet monkeys. The maintenance of a disease-free course of SIV infection in AGM likely depends on a number of mechanisms. There are a number of distinctive features of natural infection with SIV including: 1) low numbers of CD4+ T cells, and 2) low expression of CCR5 at mucosal sites. Since AGM characteristically express very low levels of CCR5 on CD4+ T cells, it is possible that SIVagm this may be a common theme among natural hosts species. We have initiated studies using adoptive transfer of purified CD4+ and CD8 alpha+ T lymphocytes from SIV-infected and uninfected AGM to an immunosuppressed mouse model (NSG) to study the fate of these cells in a controlled environment. ROLE OF THE CYTOPLASMIC TAIL OF SIV IN NEUTRALIZING ANTIBODY SENSITIVITY. An incomplete understanding of native human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) envelope glycoproteins (Envs) impedes the development of structural models of Env and vaccine design. This shortcoming is due in part to the low number of Env trimers on virus particles. For SIV, this low expression level can be counteracted by truncating the cytoplasmic tail (CT) of Env. CT truncation has been shown to increase Env incorporation into the virion and is commonly used in vaccine and imaging studies, but its effects on viral antigenicity have not been fully elucidated. To study the effects of a CT truncation of Env in viruses in similar genetic contexts, we introduced stop codons into the CT of a SIVsmE660 molecular clone and two neutralizing antibody (NAb) escape variants. These viruses shared 98% sequence identity in Env but were characterized as either tier 1 (sensitive to neutralization), tier 2 (moderately resistant to neutralization), or tier 3 (resistant to neutralization). However, the introduction of premature stop codons in Env at position Q741/Q742 converted all three transfection-derived viruses to a tier 3-like phenotype, and these viruses were uniformly resistant to neutralization by sera from infected macaques and monoclonal antibodies (MAbs). These changes in neutralization sensitivity were not accompanied by an increase in either the virion Env content of infection-derived viruses or the infectivity of transfection-derived viruses in human cells, suggesting that CT mutations may result in global changes to the Env conformation. Our results demonstrate that some CT truncations can affect viral antigenicity and, as such, may not be suitable surrogate models of native HIV/SIV Env.I